Method of making raster pattern magnetoresistors

ABSTRACT

A TECHNIQUE FOR CREATING A RASTER PATTERN MAGNETORESISTOR DEVICE FROM AN ELECTRON BEAM-RECRYSTALLIZED INSB FILM BY EMPLOYING A PHOTOLITHORGAPHIC PROCESS TO ETCH OUT OF THE INSB FILM A SUITABLE PATTERN ON WHICH MICRONSIZE INDIUM OR COPPER STRIPES ARE LATER ETCHED OUT OF A SUPERPOSED METALLIC FILM APPLIED ON THE INSB BY VACUUM DEPOSITION. THE RASTER PATTERN MAGNETORESISTOR DEVICE IS A REISTOR, WHOSE INITIAL RESISTANCE IN ZERO MAGNETIC FIELD IS BETWEEN 10 AND 1000 OHMS, AND WHOSE RESISTANCE INCREASES WITH AN APPLIED MAGNETIC FIELD, REACHING A VALUE GREATER THAN A FACTOR OF 10 OF THE INITIAL RESISTANCE IN A MAGNETIC FIELD OF 10K OE.

y 13, 1971 D. A. COLLINS ETAL 3,592,708

METHOD OF MAKING RASTER PATTERN MAGNETORESISTORS Filed July 26, 1968 In OR Cu InSb GLASS OR NONCONDUCTING METAL OXIDE SUBSTRATE FIGJ MAGNETORESISTANCE (AR/R koe DAVID A. COLLINS HARRY H. WIEDER INVE TORS 72.4 pm LINES O.l5 0.20 0.2 LENGTH TO WIDTH RATIO (.Q/w)

ATTORNEY United States Patent 3,592,708 METHOD OF MAKING RASTER PATTERN MAGNETORESISTORS David A. Collins, Ontario, and Harry H. Wieder, Riverside, Calif., assignors to the United States of America as represented by the Secretary of the Navy Filed July 26, 1968, Ser. No. 748,069 Int. Cl. C23f 17/00; H011 7/50 U.S. Cl. 156--17 4 Claims ABSTRACT OF THE DISCLOSURE A technique for creating a raster pattern magnetoresistor device from an electron beam-recrystallized InSb film by employing a photolithographic process to etch out of the InSb film a suitable pattern on which micronsize indium or copper stripes are later etched out of a superposed metallic film applied on the InSb by vacuum deposition. The raster pattern magnetoresistor device is a resistor, whose initial resistance in zero magnetic field is between 10 and 1000 ohms, and Whose resistance increases with an applied magnetic field, reaching a value greater than a factor of 10 of the initial resistance in a magnetic field of 10K oe.

The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The purpose of the invention is the production of resistors whose value is a function of the applied magnetic field, i.e., magnetoresistors. Materials best suited for the construction of magnetoresistors are the intermetallic compounds InSb and InAs. In order to produce high sensitivity magnetoresistor, i.e., devices whose resistance shows a strong variation with magnetic field, the Hall voltage generated in the semiconducting material (InSb or InAs) must be short circuited. This can be accomplished by an actual physical short circuit of the electrodes, such as in a Corbino disc, with the electrodes disposed in a coaxial fashion, or alternatively, conductive lines applied to a rectangular device can be used to produce an electrostatic short circuit. Still another procedure is the introduction of metallic inclusions into the semiconductor; however, for best results these inclusions should be arranged in a highly directional manner and should have a needle-like shape. This orientation should be normal to the applied magnetic fields and current. Magnetoresistors made of bulk InSb have a higher magnetoresistance (AR/R than 'film magnetoresistors. However, their effective resistance R(H) is small, at most a few hundred ohms. In many applications it is desirable to have the effective resistance R(H) of the order of thousands of ohms. This can best be done by means of thin film magnetoresistors.

Thin film magnetoresistors made of the two-phase system InSb and In, i.e., InSb with In inclusions, have a relatively high magnetoresistance. It is difiicult, however, to control their impurity concentration. The latter determines the ultimate temperature sensitivity of the devices. It is also difficult to produce reproducibly the same effective resistance R(O) and magnetoresistance (AR/R The method of fabrication described herein provides means for producing an electrostatic short circuit of an InSb film by photolithographic procedures.

3,502,708 Patented July 13, 1971 The advantages of the present invention are: Higher quality InSb recrystallized by an electron beam microzone process can be produced under reproducible and controlled conditions (donor impurities can be introduced into the film in accordance with copending U.S. patent application Ser. No. 747,511, filed July 25, 1968 by Harry H. Wieder and Arthur R. Clawson for Sulfur Doped Recrystallized InSb Films); a metallic high density, line-raster pattern can be used to cover the films in order to obtain a specific desired magnetoresistance ratio in terms of the length to width ratio of the InSb segment included between two raster lines. The elfective magnetoresistance (AR/R is that of the (near perfectly) short circuited InSb segment and can be used as a design criterion for various magnetoresistor configurations on either fiat or curved substrates.

Other objects and many of the attendant advantages of this invention will become readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is an illustration of a typical magnetoresistor made of electron beam microzone-crystallized InSb film covered by an In or Cu film raster pattern.

FIG. 2 shows magnetoresistance (AR/R as a function of the magnetic field H for various length to width (l/w) ratios of raster pattern lines.

The galvanomagnetic properties of 3 to 5 ,um. thick InSb films grown by electron beam microzone synthesis and crystallization of composite vacuum-deposited (In+Sb) layers are essentially the same as those of bulk InSb with a comparable impurity concentration. The magnetic field and geometry dependence of the magnetoresistance (AR/Ru), 0f raster plate magnetoresistor configurations made of such films have been investigated. A large transverse (AR/R is to be expected from a seriesconnected array of identical InSb parallelipipeds whose thickness is slight with respect to their other dimensions and whose main surfaces are covered by metallic electrodes. A somewhat smaller (AR/R can be obtained by vacuum depositing or electroplating a raster of metallic lines on an InSb plate in such manner that the InSb strips included between the lines have a small length to width (l/w) ratio in the direction of the. applied current.

Photolithographic techniques are used to construct typical raster pattern InSb film magneto-resistors such as the one shown in FIG. 1. Indium films 0.5 pm. in thickness are vacuum deposited on electron beam-crystallized InSb films deposited on glass or Pyrex substrates. A raster pattern of identical lines is then etched out of the In film by means of an etchant which attacks preferentially the In film but not the underlying InSb layer (a suitable etching solution is H PO -H O in a 40:1 ratio held at a temperature of C.). The magnetic field and geometry dependence of (AR/R of such raster plates is similar to that of single crystal InSb dendrites; magnetoresistance (AR/R is proportional to the square of the magnetic field, H up to -2K 0e, and is linear in magnetic field H above 3K oe. It increases with the electron mobility, a and with the increasing density of the In lines, i.e., with a decrease in the (l/w) ratio.

FIG. 2 shows that the magnetoresistance (AR/R of raster pattern magnetoresistors such as shown in FIG. 1 is a linear function of the length to width (l/w) ratios of raster pattern lines for (l/w)g0.2, T=296 K. and H=10K oe. Width of the metallic lines is identical to 6 spacing between lines. The thickness d and the galvanomagnetic coefficients of the respective InSb films are:

Average values of (AR/R and standard deviations are shown for 9 specimens of 42.4 nm. raster magnetoresistors. Triangular points on ordinate represent Corbino disc magnetoresistance measured in H =10K oe. Corbino disc configurations, made of InSb films with the same thickness, electron mobility and donor concentration as those of corresponding raster pattern magnetoresistors, are shown in FIG. 2 to have (AR/R in fair agreement with the extrapolated (AR/R Vs (l/w) function to (l/w)=0. In contrast with two-phase (InSb-l-In) ordered dendritic film magnetoresistors which exhibit an anomaly in the temperature dependence of (AR/R because of the anomalous temperature dependence of ,u raster pattern magnetoresistors have been found to be independent of temperature between 296 K. and 77 K. to within 2.3%.

An advantage of film over bulk magnetoresistors is their high resistance per film surface area. Small surface areas are desirable in order to obtain a high effective magnetic induction, by focusing an applied magnetic flux,

by means of flux concentrators, onto a magnetoresistor.

complex shapes on either plane or curved substrates. They can also be made in noninductive configurations such as required for high frequency applications.

Exact theoretical solutions for the geometry-dependent transverse (AR/R have been developed (H. J. Lippman and F. Kuhrt, Z. Naturforsch, 13:1, 462 (1958)); J. Haeusler, Z. Naturforsch, 17a, 506 (1962)); they are based on a conformal representation of the electric field distribution in rectangular plates. Approximate theoretical solutions have also been devised (J. P. Newsome, Proc. Inst. Electron. Eng. 110, 653 (1963); S. Gruetzmann, Solid-State Electron, 9, 409 (1966)) using finite difference equations to solve Laplaces equation in two dimensions. In each case, it is assumed that the medium is homogeneous and isotropic, that (in H=0) the current streamlines emerge normal to the equipotential electrodes,

that the constraints XT=0, -7=0 and E= are applicable, and that the resistivity acquires a gyrotropic character in an applied magnetic field. Here I is the electric field, the scalar potential and T the current density vector.

Considering a raster plate in a Cartesian coordinate system, the indium electrodes are in the xy-plane, parallel to the InSb film surfaces. The current density vector T has J as well as l and 1 components. The electric field distribution in the presence of a magnetic field is:

where H u H, is the Hall angle. A charge density w is present such that Only if zr =0 is the two-dimensional solution of Laplaces equation applicable.

The results of a resistance-paper analog plot made of the potential distribution along the cross-section of a raster plate in the vicinity of an electrode indicated that only for x d are the equipotentials a function of 2. For x d the current streamlines and equipotentials behave as if the electrodes were effectively normal to the film surfaces and the two-dimensional solution of Laplaces equathe function (w/l) -g(l/w)- ll; thus for magnetic field H210 oe., (AR/R )=11.5. The small discrepancy between the calculated and measured (AR/R is not unreasonable in view of the fact that (2d/l)z0.l9. Similar results have also been obtained on the other magnetoresistors. High raster line densities are desirable in order to decrease the (I/w) ratio and thus to increase the magnetic field sensitivity, 6(AR/R )8H of magnetoresistors, as well as for increasing their effective resistance per unit surface area. A raster consisting of 1 m. In lines is well within the resolution attainable at the present time by means of standard photolithographic techniques. However, the distortion of the equipotential lines can be significant and lead to a reduction in (AR/R unless the film thickness d is also reduced. A reduction in film thickness to less than 1 m. does not appear desirable since there is a sharp reduction in ,u with d which may be due to scattering of electrons from the film surfaces.

The photolithographic process used to produce raster pattern InSb film magnetoresistors consists of several steps as follows: First, a chemical photoresist coating (e.g., AZ-1350, Shipley Co. Inc., Los Angeles, Calif.) is applied to a previously processed InSb film, on a glass substrate, grown by electron beam microzone synthesis and crystallization. Then a particular magnetoresistor pattern mask is superposed on the coated film and the slide assembly is exposed to ultraviolet light. The exposed photoresist is then removed by the use of a developer (e.g. Developer for AZ-1350 Photoresist). Chemsol D or aqua regia is then used to etch the pattern out of the InSb film, and, then the remaining photoresist is removed with acetone. Following this an indium film 0.5 to 2 m. thick is evaporated onto the entire glass substrate, including the processed magnetoresistor pattern. A photoresist coating, the same as in the first step, is again applied to the entire slide assembly. Now a raster line pattern mask is superposed on the substrate and exposed to ultraviolet light, and the exposed photoresist is then removed using a developer as aforementioned. The line pattern is then etched out by immersing the slide assembly for 2 to 5 minutes, into a :1 solution, for example, of H PO H O held at a temperature of C.; and the remaining photoresist is removed with acetone. The indium film does not adhere to the bare glass in the hot etching solution just described; nor does the etchant attack the InSb film, only the exposed In lines of the In raster pattern. Thus a raster of In lines is left on the InSb magnetoresistor pattern (FIG. 1).

The glass substrate surrounding the magnetoresistor pattern is cut away, for example, by means of a stream of abrasive particles propelled by a nitrogen jet, such as an S. S. White Airbrasive Unit. Electrodes of the magnetoresistor can be copper electroplated and leads can be attached by soldering or by the use of a conductive epoxy cement.

Other semiconducting films such as InAs or ternary compound films such as InSb,,As can be produced by chemical vapor phase transport procedures.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within practiced otherwise than is specifically described.

What is claimed is:

1. A process for producing high raster line density pattern magnetoresistor Whose initial resistance in zero magnetic field is between and 1000 ohms and whose resistance increases with an applied magnetic field reaching a value greater than a factor of 10 of the initial resistance in a magnetic field of 10K oe., comprising:

(a) growing a semiconducting film of semiconductor material selected from the group consisting of InSb, -InAs and the ternary compound InSb,,As on a non-conducting substrate surface by electron beam microzone synthesis and crystallization,

(b) etching out a magnetoresistor pattern from said semiconducting film by photoetching means, leaving only semiconducting film in a magnetoresistor pattern on said nonconducting substrate,

(c) vacuum depositing a metallic conducting film from the group consisting of In or Cu onto the etched magnetoresistor pattern and said non-conducting substrate,

(d) coating said metallic film with a chemical photoresist material,

(e) superposing a raster line pattern mask over said photoresist coating and exposing unmasked portions of said photoresist coating to ultraviolet light, then removing the exposed portions of said photoresist coating by use of developer means,

(f) etching out the raster line pattern oly from said metallic film with etching solution which does not attack said semiconducting film for 2 to 5 minutes at 80 C., said etching solution which will not attack said semiconducting film being a 40:1 solution of H PO H O (g) removing remaining portions of said photoresist coating from remaining portions of said metallic film with a solvent, thus leaving a raster of metallic film lines on said semiconducting magnetoresistor pattern.

2. A process as in claim 1 wherein said substrate surrounding the magnetoresistor pattern is cut away.

3. A process as in claim 1 wherein said metallic film is 0.5 to 2 p.111. in thickness.

4. A process as in claim 1 wherein said magnetoresistor pattern is etched from said semiconducting film using aqua regia.

References Cited 

